For the 3 people who might care about the distinction (e.g. vuln DBs
who exclude path disclosure), this appears to be a custom error
message, not one generated by PHP, and therefore a design choice by
the developer.

Notice that in the URL above, the "_task" parameter has the value
"ma`il" - an invalid value.

roundcube appears to perform some validation of the value, then giving
a verbose error message when the validation fails.